1. Field of the Invention
The invention relates to corrosion preventing circuits for a switch, and more particularly to a corrosion preventing circuit for a switch which allows a large current switch carried on a vehicle or the like to be applied to a lower power system such as an electronic unit.
2. Prior Art
FIG. 5 is a circuit diagram showing connection of a switch generally used for opening and closing large current (e.g., 500 mA, 1 A, etc.) carried on a vehicle or the like. In FIG. 5, reference numeral 1 designates a large current switch; 2, a battery serving as a dc power supply that supplies a power supply voltage to a load, whose voltage is, e.g., 12 V; and 3, a 12 W lamp as the load.
The switch 1 that is carried on a vehicle in current use is designed so that a comparatively large current such as 500 mA or 1 A can flow therethrough. Therefore, the contact of such switch 1 is not plated, but is made of a copper in many cases as far as the treatment and material of the contact thereof is concerned.
Switches such as this which is suitable for making and breaking large current have the shortcoming that oxide films are formed at the contact of the switch and that the contact resistance of the contact tends to increase. However, it is difficult for the switches handling large current to destroy the oxide films formed at the contacts thereof by applying a large current such as about 1 A while the switch is being closed or opened.
Amid rapid development in unitizing electronic devices using LSIs (Large-Scale Integrated circuit) into modules not only in the field of vehicles but also in other fields, the signal input stage of such electronic module still uses existing mechanically strong power switches in many cases.
FIG. 6 is a circuit diagram showing connection when a large current switch such as described above is applied to an electronic unit. In FIG. 6, reference numerals 1 to 3 designate the same parts and components as those in FIG. 5; and 4, designates an electronic unit. The electronic unit 4 includes a resistor R10 for pulling up or down an input of an interface circuit (not shown); and a resistor R11 for connecting the node between the resistor R10 and the switch 1 to the interface circuit (not shown).
In the thus constructed electronic unit 4, it is required that such a small current as to be considered equivalently a signal be applied to the switch 1, and the value of current to be applied to the contact of the switch 1 is determined by the pull-up or pull-down resistor R10 of the electronic unit 4. The current value, being slightly different from one electronic unit to another, is determined by the capacity of the electronic unit 4 or of the pull-up or pull-down resistor R10. The value is in the order to several mA to 10 mA.
As described above, if a large current switch carried on a vehicle or the like is used for the signal input stage of a small current system such as an electronic unit, then a current flowing through the switch takes a small value defined by the electronic unit. Therefore, if an oxide film is formed at the contact of the switch 1, the contact resistance of the switch 1 is increased, thereby imposing the problem that the electronic unit cannot detect the state of the switch 1.
What is required to protect the contact of the switch 1 is to destroy the oxide film formed at the contact of the switch. However, to apply a large current, the capacity of the pull-up or pull-down resistor R10 must be 2 W or more. A pull-up or pull-down resistor R10 having a large capacity is disadvantageous in terms not only of cost of the resistor, but also of construction of the circuit. That is, a cooling means must be involved to control heating of the resistor, which is a bottleneck from the viewpoint of downsizing the electronic unit.